Step 1: Prepare the Pistachio Biscuit

1. Preheat Oven: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. Mix the Ingredients: In a bowl, whisk together eggs and sugar until the mixture is pale and fluffy. Stir in melted butter, pistachio paste, vanilla extract, and a pinch of salt.
3. Add Dry Ingredients: Sift together flour and baking powder, then fold it into the wet ingredients until just combined.
4. Bake the Biscuit: Spread the batter evenly on the prepared baking sheet and bake for 15-20 minutes, or until a toothpick inserted comes out clean. Let it cool completely.

Step 2: Make the Vanilla Cream

1. Heat the Milk and Cream: In a saucepan, heat the milk and heavy cream over medium heat until warm (but not boiling).
2. Whisk the Egg Yolks: In a separate bowl, whisk together the egg yolks, sugar, and cornstarch until smooth.
3. Temper the Eggs: Slowly pour the warm milk mixture into the egg yolks while whisking constantly to avoid curdling.
4. Cook the Cream: Return the mixture to the saucepan and cook over low heat until thickened. Stir in vanilla extract and a pinch of salt. Let it cool to room temperature.

Step 3: Make the Pistachio Praline

1. Caramelize the Sugar: In a small pan, combine sugar and water. Heat over medium heat, swirling the pan occasionally until the sugar dissolves and turns a light amber color.
2. Add Pistachios: Add pistachios to the caramel and stir until they’re evenly coated. Pour the mixture onto a baking sheet lined with parchment paper and let it cool completely.
3. Crush the Praline: Once cooled, break the praline into small pieces using a rolling pin or food processor.

Step 4: Prepare the Pistachio Mousse

1. Heat the Milk and Pistachio Paste: In a saucepan, heat milk and pistachio paste together until warm.
2. Whip the Cream: In a separate bowl, whip the heavy cream with powdered sugar until soft peaks form.
3. Add Gelatin: Stir in the dissolved gelatin into the warm milk-pistachio mixture, then fold in the whipped cream until smooth and fluffy.

Step 5: Assemble the Entremets

1. Layer the Pistachio Biscuit: Cut the cooled pistachio biscuit into a circle that fits the base of your mold. Place it at the bottom of a round cake ring or mold.
2. Add the Pistachio Mousse: Spoon half of the pistachio mousse on top of the biscuit, smoothing it out with a spatula.
3. Layer with Vanilla Cream: Spread a layer of vanilla cream over the mousse and top it with pistachio praline.
4. Repeat Layers: Add another layer of pistachio mousse, followed by the remaining vanilla cream.
5. Chill: Let the entremets chill in the refrigerator for at least 3 hours, or until set.

Step 6: Apply the Mirror Glaze

1. Make the Glaze: In a saucepan, combine water, sugar, and condensed milk. Bring to a boil and stir in the gelatin and white chocolate. Add food coloring if desired.
2. Cool the Glaze: Let the glaze cool slightly, then pour it over the chilled entremets, ensuring it covers the entire surface evenly.
3. Chill Again: Return the entremets to the fridge for another hour to set the glaze.